The invention is directed to a process for the conditioning of radioactive and toxic wastes for transportation and final storage in which waste is bound in a matrix constructed on a basis of carbon and molded into solid blocks.
In the nuclear art there are accumulated radioactive wastes in liquid and solid form in the fuel recycling industry, the nuclear power plants and reprocessing. Distinctions are made in regard to activity as high, average and weakly active wastes. The highly active waste with an activity of greater than 10.sup.4 Ci/cm.sup.3 is formed preponderantly in the reprocessing of nuclear fuels. The average active waste with an activity of 10.sup.4 to 10.sup.-2 Ci/cm.sup.3 are produced both in the reprocessing as well as in the final recycling industry and in power plant operation.
In order to reduce the stored volumes, customarily the liquid radioactive waste is first concentrated by evaporation and then solidified. For the solidification of highly radioactive waste several processes are known.
Preferably the solidification takes place by calcining in a fluidized bed between 350.degree.-900.degree. C. Thereby there is obtained a mixture of non-volatile oxides and metal components, which can be present as powder or granulate. For safe final storage the powder or granulate is bound into a glass-like matrix and in connection therewith converted into solid blocks.
For the fixation of average and weakly active wastes processes are known according to which the waste material together with e.g. bitumen is heated and subjected to an extrusion process. Thereby the radioactive waste is bound into the bitumen composition and after cooling the solidified composition in transportation casks final stored.
Besides processes were developed according to which the radioactive waste is fixed by cementation. Customarily in these processes the radioactive waste is processed in the form of a sludge which is composed of about 80 weight % of liquid and the residual 20% of solid components. The sludge is mixed with cement for the production of concrete and solidified at room temperatures. The production of concrete optionally can be taken up directly in the transportation tanks for the final storage.
Furthermore processes are known for the fixing of radioactive waste in which the waste is mixed into a polymerizable resin preferably at room temperature and this resin is polymerized with monomers to a solid block.
The previously known processes have a series of disadvantages. The vitrification of the waste takes place at very high temperatures which customarily are above 1000.degree. C. Therefore the process is expensive and dear. The heat conductivity of the glass matrix is low. In order not to exceed an impermissibly high central temperature of the blocks because of the production of heat in the decay of the fission products, the waste concentration and the block diameter is limited to relatively small values. Since the coefficients of thermal expansion of glass and container material are very different there occur in the cooling thermal stresses which can lead to undesired stress corrosion. Besides the cooling time required is very long and can amount to several days.
The bitumenization is a hot process. At relatively high temperatures required for extrusion because of a possible danger of fire there must be taken strong safety precautions which are industrially expensive, susceptible to disturbances and therewith dear.
The substantial disadvantages of the inclusion in cement are a large volume of waste, the frequent poor setting property of the cement in contrast to the included waste solutions, and undesired leaching out of the radioactive waste enclosed in the concrete caused through porosity.
In the binding in the polymerizable synthetic resin it is a matter additionally of hydrocarbon compounds. Therefore there is no guarantee of the retaining capacity for the active gas tritium. Besides through the aging process the brittleness of the synthetic resin can increase and therewith the mechanical integrity of the blocks is endangered.
It is known from DeBacci U.S. Pat. No. 3,994,822 to enclose radioactive waste in a matrix of .beta.-silicon carbide by "overcoating" the waste particles with silicon carbide and graphite, pressing to a porous shaped article and impregnating with liquid silicon, during which .beta.-silicon carbide is formed. However, this production of matrix is very expensive and the silicon carbide only has a relatively poor heat conductivity.
According to Bunnell U.S. Pat. No. 3,993,579 radioactive waste is encapsulated in vitreous carbon by covering the particles with a resin and heating to 1000.degree. C. for 150 hours. Here also there is a cumbersome encapsulation process whereby there are producible only thin carbon layers and there is needed an additional metal matrix and a storage container.
Therefore it was the problem of the present invention to provide a process for the conditioning of toxic, especially of radioactive waste for the transportation and the final storage through enclosure in a carbon matrix, which avoids the disadvantages of the known processes and particularly permit the solidification of the waste to non-burnable blocks with higher mechanical and chemical stability, better resistance to extraction, higher heat conductivity and with high resistance to rays of any kind.